1. Field of the Invention
The invention is based on a process for producing a workpiece from an alloy containing dopant and based on titanium aluminide.
High-temperature alloys for heat engines based on the intermetallic compound TiAl which are suitable for producing cast and forged components and which are capable of supplementing and in part replacing the conventional nickel-based superalloys.
The invention relates to the melting and casting of alloys produced from the intermetallic compound TiAl and doped with further additives and to the thermal and thermomechanical further processing to produce usable workpieces having good mechanical properties.
2. Discussion of Related Art
Intermetallic compounds of titanium with aluminum have some interesting properties which make them appear attractive as structural materials in the medium and higher temperature range. These include, inter alia, their low density compared with superalloys, which reaches only approximately 1/2 of the value for Ni superalloys. An obstacle to their technical usability in the present form is, however, their brittleness. The former can be improved by additives, with higher strength values also being achieved at the same time. Possible intermetallic compounds which are known as structural materials and have already been introduced in part are, inter alia, nickel aluminides, nickel silicides and titanium aluminides.
Attempts have already been made to improve the properties of pure TiAl by slightly altering the Ti/Al atomic ratio and by adding other elements by alloying. Further elements which have been proposed, for example, are, alternatively, Cr, B, V, Si, Ta, and also (Ni+Si) and (Ni+Si+B), and furthermore Mn, W, Mo, Nb and Hf. The intention was, on the one hand, to reduce the brittleness, i.e. to increase the malleability and toughness of the material and, on the other hand, to achieve as high a strength as possible in the temperature range of interest between room temperature and working temperature. In addition, a sufficiently high oxidation resistance was required. These objectives were, however, only partly achieved.
The high-temperature strength of the known aluminides, however, still leaves something to be desired. In accordance with the comparatively low melting point of these materials, the strength, in particular the creep strength in the upper temperature range is inadequate, as also emerges from publications in this connection.
Furthermore, the molding of intermetallic phases based on titanium aluminides presents certain problems. The high affinity of the elements involved for oxygen, in particular that of titanium, makes the production of moldings by casting difficult. Poor mold filling capacity, porosity and shrinkage cavities are the consequences. In addition, the properties of the as-cast structure cannot be improved to the desired extent by subsequent heat treatment. An obstacle to conventional hot deformation, on the other hand, is the comparatively imperfect ductility in the lower temperature range.
The following documents are cited in relation to the prior art:
N. S. Stoloff, "Ordered alloys--physical metallurgy and structural applications", International metals review, Vol. 29, No. 3, 1984, pages 123-135. PA0 G. Sauthoff, "Intermetallische Phasen" ("Intermetallic Phases"), Werkstoffe zwischen Metall und Keramik, (Materials between metal and ceramic) Magazin neue Werkstoffe 1/89, pages 15-19. PA0 Young-Won Kim, "Intermetallic Alloys based on gamma Titanium Aluminide", JOM, Jul. 1989. PA0 U.S. Pat. No. 4,842,817 PA0 U.S. Pat. No. 4,842,819 PA0 U.S. Pat. No. 4,842,820 PA0 U.S. Pat. No. 4,857,268 PA0 U.S. Pat. No. 4,836,983 PA0 EP-A-0,275,391
The properties of the known modified intermetallic compounds and their conventional processing methods still do not in general satisfy the technical requirements in order to produce usable workpieces from them. This applies, in particular, in relation to the high-temperature strength and the toughness (ductility). There is therefore a need for further development and improvement of such materials and their molding, and also the beneficial influencing of the mechanical properties of the workpieces produced from them.